1. The Harm of Diesel Engine Damage
Diesel engine damage, often referred to as "burned tiles," is a common issue during testing and operation of diesel engines. When such a failure occurs, the repair process can be both time-consuming and costly. For minor cases, the locomotive must replace the bearing bush, which increases workload and may lead to inconsistent repair quality. In more severe situations, the entire diesel engine has to be removed from the locomotive for disassembly and repair. If the bearing bush is burned, it can cause deformation and discoloration of the crankshaft journal and main bearing housing, requiring replacement of multiple parts, including the bearing bush, crankshaft, and engine block. This results in extended repair times and significantly increased maintenance costs.
The diesel engine is the core component of a diesel locomotive, responsible for providing power and traction. If the engine fails while in operation, it can lead to major accidents, causing significant economic and reputational losses for the manufacturer and disrupting railway operations. To prevent such failures, manufacturers have implemented various preventive measures. However, due to the complex causes of engine damage, it remains difficult to completely eliminate the risk. Therefore, understanding how to repair damaged engine bodies is crucial for minimizing financial impact and ensuring continued serviceability.
2. Structure of the Main Bearing and Accuracy Analysis of the Engine Block
The engine block serves as the foundation of the diesel engine, with the main bearing holes formed by assembling the main bearing housing and cover using bolts. Once the bearing bush and crankshaft are installed, the main bearings are formed. These bearings are open-closed sliding bearings made of two semi-circular alloy segments. They rely on pressure lubrication to form an oil film that supports the crankshaft and ensures smooth operation.
The engine block is typically made of high-strength ductile iron, with extremely tight tolerances on the main bearing holes. These holes serve as reference points for other critical features like camshaft and cylinder bores. Modern high-power, low-emission diesel engines require precise machining, with each main bearing hole having a diameter of 265+0.0320 mm, cylindrical tolerance of 0.025 mm, and straightness of 0.03 mm over full length. Machining is done using a gantry machining center, and precision is verified through collimators, bridge gauges, and coordinate measuring machines to ensure compliance with design specifications.
3. Condition of the Main Bearing Holes After Engine Damage
When the crankshaft operates within the main bearing hole, if the oil film thickness becomes insufficient due to mechanical or operational issues, the bearing may lose its load-bearing capacity. This leads to direct contact between the crankshaft and the bearing surface, resulting in semi-dry or dry friction. Such conditions cause rapid wear and overheating, potentially melting the bearing material and damaging the crankshaft and engine block. Post-damage measurements show that the main bearing holes may shrink, become out of round, or lose straightness, while the bearing caps may become stuck or deformed.
4. Repair Methods for the Engine Block After Damage
Since the main bearing holes serve as reference points for other key features, maintaining their accuracy is essential. One of the most effective methods to restore them is by replacing the main bearing caps and re-machining the holes. The main bearing cap is a weak and highly stressed component, making it prone to deformation after damage. Replacing it allows for correction of the center position, which is critical for the alignment of other components like camshaft and cylinder holes.
The repair process involves: (1) disassembling the engine, cleaning the block, and removing all accessories; (2) reassembling the bearing caps according to specifications; (3) performing three-axis precision measurements; (4) replacing damaged caps with new ones that have a 2.0 mm machining allowance; (5) calibrating the machine tool to correct the center positions of the main bearing holes, controlling errors within 0.01 mm; and (6) manually and instrumentally inspecting the repaired block before final assembly.
5. Inspection After Repair
Post-repair inspection is crucial to ensure the engine block meets all required standards. Manual checks include measuring the inner diameter at three points and checking surface roughness. Three-coordinate measurement assesses the diameter, cylindricity, and straightness of the main bearing holes, as well as the accuracy of adjacent features. A pressure tile inspection is also performed to check the contact area between the bearing bush and the hole, ensuring at least 80% contact for proper fit and function.
6. Conclusion
Ensuring the center position of the main bearing holes is the key to successful engine block repair. Replacing the main bearing caps is the most efficient method. For severe damage, coating or welding repairs may be necessary, but these require careful control of heat and stress to prevent further deformation. After repair, the engine block must be thoroughly cleaned and inspected to ensure no internal debris or defects remain. Proper attention to cleanliness and surface condition is essential to maintain accurate alignment and long-term performance.
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